Reactive oxygen species (ROS) are endogenously produced oxidants with various functions ranging from host defense to signaling. These transient species can cause severe damage to the body when their production is dysregulated or when environmental factors elevate their concentrations. To study their effects and prevent oxidative harm, tools capable of monitoring ROS in cells and tissue in a sensitive and selective fashion are required. In this Review, a summary of existing ratiometric probes is provided, together with a critical discussion of selected examples. Bright ideas: Reactive oxygen species (ROS) are endogenously produced oxidants with various functions ranging from host defense to signaling. To study their effects and prevent oxidative harm, tools capable of monitoring ROS in cells and tissue in a sensitive and selective fashion are required. In this Review, a summary of existing ratiometric probes is provided, together with a critical discussion of selected examples.